In a small bowl, place yeast, 1 teaspoon sugar, and lukewarm water. Allow to sit around 10 minutes, until it becomes foamy on top.

In a large bowl, or stand mixer fitted with the whisk attachment, mix together 1½ cups flour, salt, cinnamon, vanilla, and sugar. After the water-yeast mixture has become foamy, add to flour mixture along with oil. Mix thoroughly. 

Add another 1 cup of flour, eggs, and 1 egg yolk; mix until smooth. Switch to the dough hook attachment if you are using a stand mixer.

Add another 1½-2 cups of mixed flour, mixing thoroughly, then remove from bowl and place on a floured surface. Knead remaining ½ cup flour into dough, continuing to knead for around 5 minutes. 

Place dough in a greased bowl and cover with damp towel. Allow to rise around 3 hours, punching down at least once, if possible.

Preheat oven to 350°F.

Mix 1 cup sugar and 2 teaspoons cinnamon in a large bowl. 

Using a dough cutter or sharp knife, cut dough into 1-inch balls. Place the dough balls into the sugar-cinnamon mixture until they are completely coated. Spread the dough balls evenly in a greased bundt pan. Alternatively, you can bake in muffin tins to make mini versions of monkey bread.

In a saucepan over medium-high heat, melt the two sticks of butter or margarine with the brown sugar until the sugar is completely dis-solved and the color is a rich amber. Whisk in the vanilla and poppy seed filling. Make sure to whisk vigorously so that there are no clumps of poppy seed.

Pour mixture over the challah dough. Bake for 35 minutes, until the top is browned. Remove from oven and allow to cool 15-30 minutes in the pan. Separate the dough from the pan gently using an offset spatula. Flip over onto a plate to remove from pan.